Fix remaining DExpr ownership regressions

[stevenvar]

Summary

Fix a few remaining regressions introduced by the DExpr ownership cleanup in that were not covered by the later follow-up fixes.

What this does

• preserve missing expression slots instead of rewriting them into constants
• restore the older selective reshape expression refresh behavior in Grappler shape refinement
• restore gating for TF<->XLA shape expression propagation
• distinguish missing expression slots from real unknown expressions using a dedicated missing-expression sentinel

Notes

This PR is intentionally narrow: it keeps the ownership model, and only reverts the behavioral drift that looked unrelated to the ownership migration itself.